In a motor vehicle system with a remote control unit, the latter has a transmitter which functions as a key and is provided with a coding device. The receiver on the vehicle and connected with the door locks has a complementary decoder and communication between the two units can operate by high-frequency modulation so that commands are transmitted in a so-called communication protocol, are received in that protocol and converted to door-lock operations.
The transmitter and receiver operate in a given frequency band. Each communication protocol can comprise, inter alia, a starting bit, a fixed code portion which can coordinate the transmitter of the key with the receiver of the vehicle and a changeable code portion. The communication protocol is thus comprised of bits which can be "ones" and "zeros", "high" and "low" levels or signals which can be interpreted as "yes" and "no".
In the case of frequency modulation the signal frequency may be bits representing .DELTA.f plus a carrier frequency or the carrier frequency minus .DELTA.f. .DELTA.f can be, for example, 2.5 kHz. To ensure that no information is lost, the band width must be sufficiently large and one can operate, for example, at a frequency between 433 and 435 MHz, for example, the so-called Industrial/ Scientific/Medical band or ISM band.
Central locking units of the described type can be constructed as described in DE 42 27 887 C1 and DE 42 38 858 C1. The systems of these patents have been found to be effective in practice and reference may also be had to commonly-owned copending application Ser. No. 08/098,481 filed Jul. 28, 1993.
These systems have been successful but when such systems are provided in vehicles which may be parked close enough together that their locking systems may be operated by transmitters held by their owners, there is nevertheless a problem.
This problem arises because of the close proximity of the frequencies with which communication is effected between the respective transmitters and receivers.
Furthermore, because other systems than vehicle transmitter keys are used in a given band, interference may be even more common.
There are, of course, places where communications are more secure as in the case, for example, for the military and it is known, for example, to use a spread-band technique where secret communication is desired (see for example "electronik industrie 2, 1996", pages 33 to 38). In this case, the useable frequency band is subdivided into a number of N frequency bands which are referred to as channels. A channel encompasses the carrier frequency and the frequency modulation for that carrier frequency of .+-..DELTA.f. The transmitter and receiver are synchronized to the same carrier frequency and the timing pattern switches channels, i.e. hops the channel, synchronously in the transmitter and receiver.
The spread spectrum communication technique is used not only because of the ability to transmit signals which cannot be readily intercepted, but also in cases in which communication is to be carried out in both a long distance and a close communication mode and where a variety of different signals are used. Commercially the spread spectrum technique can be used for cellular mobile communications, for cordless telephones, for global positioning systems using satellites and for so-called Personal Communication services.